A grain-oriented electrical steel sheet is a soft magnetic material used as an iron core material of transformers, generators, and the like, and has a crystal microstructure in which the <001> orientation, which is an easy magnetization axis of iron, is highly accorded with the rolling direction of the steel sheet. Such microstructure is formed through secondary recrystallization where coarse crystal grains with (110)[001] orientation or the so-called Goss orientation grows preferentially, during secondary recrystallization annealing in the production process of the grain-oriented electrical steel sheet.
Conventionally, such grain-oriented electrical steel sheets have been manufactured by heating a slab containing around 4.5 mass % or less of Si and inhibitor components such as MnS, MnSe and AlN to 1300° C. or higher, and then once dissolving the inhibitor components, and then subjecting the slab to hot rolling to obtain a hot rolled steel sheet, and then subjecting the steel sheet to hot band annealing as necessary, and subsequent cold rolling once, or twice or more with intermediate annealing performed therebetween until reaching final sheet thickness, and then subjecting the steel sheet to primary recrystallization annealing in wet hydrogen atmosphere for primary recrystallization and decarburization, and then applying an annealing separator mainly composed of magnesia (MgO) thereon and performing final annealing at 1200° C. for around 5 hours for secondary recrystallization and purification of inhibitor components (e.g. see U.S. Pat. No. 1,965,559A (PTL 1), JPS4015644B (PTL 2) and JPSS113469B (PTL 3)).
As mentioned above, in the conventional production processes of grain-oriented electrical steel sheets, precipitates such as MnS, MnSe and AlN precipitates (inhibitor components) are contained in a slab, which is then heated at a high temperature exceeding 1300° C. to dissolve these inhibitor components once, and in the following process, the inhibitor components are finely precipitated to cause secondary recrystallization. As described above, in the conventional production processes of grain-oriented electrical steel sheets, since slab heating at a high temperature exceeding 1300° C. was required, significantly high manufacturing costs were inevitable and therefore recent demands of reduction in manufacturing costs could not be met.
In order to solve the above problem, for example, JP2782086B (PTL 4) proposes a method including preparing a slab containing 0.010% to 0.060% of acid-soluble Al (sol.Al), heating the slab at a low temperature, and performing nitridation in a proper nitriding atmosphere during the decarburization annealing process to use a precipitated (Al,Si)N as an inhibitor during secondary recrystallization. (Al,Si)N finely disperses in steel and serves as an effective inhibitor. However, since inhibitor strength is determined by the content of Al, there were cases where a sufficient grain growth inhibiting effect could not be obtained when the hitting accuracy of Al amount during steelmaking was insufficient. Many methods similar to the above where nitriding treatment is performed during intermediate process steps and (Al,Si)N or AlN is used as an inhibitor have been proposed and, recently, production methods where the slab heating temperature exceeds 1300° C. have also been disclosed.
On the other hand, investigation has also been made on techniques for causing secondary recrystallization without containing inhibitor components in the slab from the start. For example, as disclosed in JP2000129356A (PTL 5), a technique enabling secondary recrystallization without containing inhibitor components, a so-called inhibitor-less method was developed. This inhibitor-less method is a technique to use a highly purified steel and to cause secondary recrystallization by controlling the textures of the steel.
In this inhibitor-less method, high-temperature slab heating is unnecessary, and it is possible to produce grain-oriented electrical steel sheets at low cost. However, this method is characterized in that, due to the absence of an inhibitor, magnetic properties of the products were likely to vary with temperature variation and the like in intermediate process steps during manufacture. Texture control is an important factor in this technique and, accordingly, many techniques for texture control, such as warm rolling, have been proposed. However, when textures are not sufficiently controlled, the degree to which grains are accorded with the Goss orientation ((110)[001] orientation) after secondary recrystallization tends to be lower compared to when utilizing techniques using inhibitors, resulting in the lower magnetic flux density.